/Engine/Source/Runtime/Core/Public/Memory/VirtualStackAllocator.h
RequestedStackSize will be rounded up to a whole number of pages and reserved One page will be earmarked as a guard, so the available memory will be NextMultipleOf(ReqestedStackSize, SystemPageSize) - SystemPageSize
FVirtualStackAllocator ( size_t RequestedStackSize, EVirtualStackAllocatorDecommitMode Mode )